Big Sur startup fails.

I have just updated my 3 Macs with Big Sur, but one is hanging.
MacBook Pro - no problem.
Older Mac mini - no problem.
This year's Mac mini - restart shows Apple logo, and progress bar at roughly 90%. It has been like this for over 3 hours. What do I do now (long time Mac user but not an expert)?
Big Sur startup fails.
 
 
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